[johncolanduoni]

> Quantum computers promise to do things that classical computers will never be able to do. A 'proper' QC with on the order of a few thousand qubits can perform calculations that even a known-universe-sized classical computer will find intractable.

Your first statement is correct, but Shor's algorithm (the one I suspect you are referring to when you talk about a universe sized computer) is not an example of one. We do not know that there is not a classical factoring algorithm that is as good or better than Shor's, and we have no quantum algorithms as of yet that can give an exponential speedup over any possible (but possible yet unknown) classical algorithm (which is needed for the universe sized computer thing). We have some that give more modest speed ups that are known to be unbeatable by classical algorithms; Grover's search algorithm is such an example, but it is only quadratic. This means you have a quantum computer of size N, you only need a classical computer of size N^2 to match it.

> Optimisation quickly becomes irrelevant when you scale these problems just a little bit more.

The problem with quantum computers isn't optimization of algorithms, but it's still optimization of strategies for dealing with errors (which sometimes look a lot like algorithms). Building reasonably accurate logic gates was figured out with a lot less effort when we started even thinking of building such things, and their reliability didn't fall off with increasing size very quickly. Our technology for quantum computers, on the other hand, all have crippling flaws as of yet. The most common one (superconducting qubits and ion trap based designs both suffer from this) is that when we try to make the computer bigger, it needs an unscalable amount of error correction an eventually stops working altogether. Some other approaches (linear optical quantum computer for example) can scale up without getting worse per se, but the gates we can build are already so unreliable that we still need too much error correction to scale. So optimizing our error correction strategies is one possible avenue that is being explored.